1. Field of the Invention
This invention involves Fabry-Perot cavities, and optical communication systems which include such cavities.
2. Description of the Prior Art
The economic advantages, envisioned years ago, of transmitting information in the form of optical signals have now been realized in commercial systems. Accordingly, designs for future optical communication systems go beyond the simple transmission of information on an optical carrier, and include the processing of signals while still in optical form. Current optical transmission systems must convert the optical signal to an electronic one before processing can take place. Such processing involves standard electronic devices. In the next generation of optical communication systems the optical signal itself often will be processed without conversion to an electronic signal. Such optical processing will require optical devices which are analogous to the electronic devices used for processing electronic signals, e.g., amplifiers, modulators, filters, etc. One aspect of this invention relates to a new type of Fabry Perot cavity which can be used to process optical signals and to a frequency shift key (FSK) modulation transmission system which uses a Fabry Perot cavity located at a receiver in combination with a laser such as a single electrode distributed feedback semiconductor laser diode.
The Fabry-Perot cavity was invented in the late 19th century. Its operation is well understood and discussed in most of the classic texts. See, for example, Born & Wolf, Principles of Optics, MacMillan, 1959, pages 378 et seq. An exemplary Fabry-Perot comprises a region bounded by two plane, parallel mirrors. The structure, as an entity, transmits only certain wavelengths, for which the cavity is said to be in "resonance"--a condition obtained by appropriately adjusting the cavity parameters. At resonance the cavity transmits a series of equally spaced wavelengths. The spacing between these wavelengths, called the "free spectral range" of the cavity (FSR), is a function of, among other parameters, the spacing between the mirrors.
The use of Fabry-Perot cavities to process optical signals, for example as filters, is well known. However, the application of such devices to the processing of optical signals in commercial communication systems has been hampered by, among other constraints, the lack of practical designs which had the proper characteristics, such as appropriate values of free spectral range. Nevertheless designs have been suggested that more closely meet the needs of a commercial system. For example, in Electronics Letters, Vol. 21, pp. 504-505 (May 12, 1985), J. Stone discussed a Fabry-Perot design in which the cavity was an optical fiber waveguide with mirrored ends. The free spectral range of the resulting cavity is determined by the length of the fiber segment, and accordingly different free spectral ranges can be obtained by using fibers of different lengths. The cavity can be "tuned" over one free spectral range by changing the cavity optical length by one-half the wavelength value of the light entering the cavity. In this way the cavity can be "tuned" to resonate at, and therefore transmit, light of different wavelength values. To obtain such tuning, the cavity length can be changed by means of an exemplary piezoelectric element attached to the fiber, which, when activated, will stretch the fiber and increase the associated cavity optical length accordingly.
Among the advantages of this "fiber Fabry-Perot" is the fact that the fiber is a waveguide. This eliminates deleterious diffraction effects present in long Fabry-Perot cavities which are not waveguides. The elimination of the deleterious diffraction effects is associated with the guiding characteristics of the fiber. However, the difficulty of working with small lengths of optical fiber precludes large values of free spectral range when using fiber Fabry-Perots, and consequently limits the usefulness of the fiber Fabry-Perot design.
Large free spectral ranges can be obtained using "gap" Fabry-Perots in which the cavity is a small gap. However, because of diffraction losses longer gap cavities are less practical, and therefore the gap Fabry-Perot is not adequate for applications which require the smaller free spectral ranges otherwise associated with larger gaps. Other techniques are known to minimize diffraction losses in large gap cavities, such as the use of expanded beams. However, those techniques involve other limitations which the practitioner may desire to avoid.
It is clear that while fiber Fabry-Perots can be used where short free spectral ranges are required, and gap Fabry-Perots can be used where large free spectral range Fabry-Perots are required, there is no effective design to answer the need for mid-range Fabry-Perots.
Another aspect of this invention relates to a frequency shift key (FSK) modulation transmission system which uses a single electrode distributed feedback semiconductor laser diode in combination with a Fabry-Perot cavity located at a receiver.
Optical communications systems are currently of commercial importance because of their ability to carry large amounts of information. Optical communication systems normally have a light source optically coupled to a photodetector via an optical fiber. Systems presently in use carry information at rates which are in excess of 100 Mbit/sec and, it is believed that future systems will carry information at very much higher rates.
For the higher transmission rates and greater distances between the light source and the photodetector, the light source currently preferred by those skilled in the art is a semiconductor laser diode. These diodes are relatively compact and can emit radiation with a relatively narrow spectral width in the wavelength regions presently of greatest interest. Diodes can now be fabricated having both single transverse and single longitudinal mode output. Such diodes are commonly referred to as single frequency lasers. These diodes are desirable in many applications because they, for example, maximize light coupled into the fiber and, at the same time, minimize the deleterious aspects of the fiber chromatic dispersion. Chromatic dispersion may broaden the light pulse which results in limiting the attainable bit rate and distance between the source and the photodetector. If either the bit rate or the distance between the source and the photodetector becomes too great, adjacent light pulses will overlap as a result of fiber dispersion and information will be lost. Normally, to avoid a loss of information due to dispersion, one or more regenerators will be inserted in the fiber between the source and the photodetector. The regenerator reconstructs the broadened, stretched-out light pulse into a more clearly defined light pulse.
Although a variety of modulation techniques are available, present systems normally use intensity modulation of the laser output to convey information. That is, information is conveyed by variation in the intensity of the light output from the laser. This system is normally referred to as Amplitude Shift Keying (ASK) System.
However, other modulation techniques offer specific advantages over intensity or amplitude shift keyed modulation. For example, higher transmission rates are possible with frequency modulation than are possible with intensity modulation for at least two reasons. First, the combination of the inherent frequency modulation or intensity modulation response with RC parasitics results in a more efficient high frequency response with frequency modulation than with intensity modulation. Second, the roll-off in response above resonance is slower for frequency modulation than for intensity modulation.
Moreover, direct intensity modulation of a semiconductor laser becomes increasingly difficult as the bit rate increases. Direct intensity modulation means that the intensity of the light output is varied by varying the current through the laser. This type of modulation has at least three problems which become significant at high bit rates. First, current modulation sufficient for intensity modulation causes large changes of the semiconductor laser diode wavelength which broadens the spectral width of the emitted radiation. This effect is commonly termed chirp and can be as large as, for example, five Angstroms. Chirp is often undesirable during intensity modulation because of the dispersive properties of the fiber. Second, intensity modulation of a laser requires a large amount of current, typically more than 60 mA which must be rapidly switched on and off. This switching becomes more difficult as the bit rate increases. Third, unless special precautions are taken, many single frequency lasers cannot be fully intensity modulated because of laser mode hopping--the laser output shifts from one longitudinal mode to another. This is commonly referred to as the "extinction ratio penalty".
Because of these reasons, alternatives to direct intensity modulation have been considered. One alternative commonly contemplated is the use of an external modulator positioned adjacent to the laser which might be, for example, an integrated optic modulator. The laser emits radiation continuously and the desired intensity modulation is supplied by signals to the modulator which vary light absorption within the modulator. Potentials normally greater than ten volts are often required for efficient operation of external modulators currently contemplated for use at high frequencies. The voltages required generally increase as the frequency increases. Additionally, there is the problem of obtaining simple, efficient, high speed modulators. There is also the additional problem of signal loss which results from the coupling between the laser and modulator as well as between the modulator and the optical fiber.
Another approach uses coherent optical techniques which require frequency locking two oscillators separated by an intermediate frequency (IF). While high sensitivity is obtained, locking the oscillators together can be difficult as they may be at diverse locations which can be as far as 100 km apart. In addition, processing of the IF signal adds complexity to the receiver.